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Nearly 2 million residents of Greater Boston lost their supply of clean drinking water when a huge pipe abruptly burst yesterday, prompting authorities to declare a state of emergency and to impose a sweeping order for homeowners and businesses to boil the untreated water now flowing from their taps.

Governor Deval Patrick said residents in Boston and 29 other communities east of Weston should boil water for at least a minute before drinking it to avoid the risk of getting sick. He also asked bottled water companies and the National Guard to help make clean water available to residents in the affected communities.

The crisis began around 10 a.m. yesterday when a 10-foot-wide pipe in Weston sprang a leak, which worsened throughout the afternoon and eventually cut off Greater Boston from the Quabbin Reservoir, where most of its water supply is stored.

The Massachusetts Water Resources Authority said it could continue supplying water by activating a backup system that began drawing water last night from the Sudbury Reservoir, and can also tap into the Weston and Spot Pond reservoirs if necessary. The backup water, which one official compared with “untreated pond water,’’ can be used for bathing and flushing toilets, but not for drinking or cooking.

Authorities said they were attempting to set up mobile units to chlorinate the backup water supply, but they cautioned that even so, the water from the backup system would not meet federal drinking water standards.

“This is everyone’s worst nightmare in the water industry,’’ said Frederick A. Laskey, executive director of the Massachusetts Water Resources Authority.

Local officials across the region were scrambling to warn residents about the potential for contamination. In Boston and other communities, police were driving up and down streets in cruisers, using bullhorns to blare boil-water warnings. At Massachusetts General Hospital, staff put up signs warning “Don’t drink the water.’’

“I ask everyone to check in on elderly or vulnerable neighbors,’’ the governor said at a press conference with Laskey at the headquarters of the Massachusetts Emergency Management Agency in Framingham. He asked people to avoid “unnecessary use of water, such as washing cars and lawn-watering,’’ and warned that drinking unboiled water could make people sick.

City officials in Boston were distributing fliers to 14,000 public housing units. City inspectors were also trying to get the word out to restaurant owners, and the Boston Public Health Commission held an emergency conference call with local hospitals, which city officials said would not experience any disruptions in medical care.

The “good news,’’ Laskey said, is that “we continue to maintain the flow for firefighting’’ and for toilets and other nondrinking purposes. Laskey said the backup system has enough untreated water to indefinitely supply the region, but supplies of bottled water were running low in many stores last night as people rushed to stock up.

Many questions remained unanswered.

Officials said they did not know how long it would take to restore clean drinking water to the region, but Laskey said he hopes it will be “days, not weeks.’’

Officials also said they did not know what caused the relatively new seven-year-old steel pipe to break 20 feet underground, near Recreation Road by the intersection of the Massachusetts Turnpike and Route 128.

Officials shut off the water supply into the pipe at about 6:40 p.m. Soon after, engineers discovered rubber gaskets floating above the leak, leading them to theorize that the pipe had ruptured at a sleeve connecting sections of the pipe.

By 10 p.m., Laskey said engineers could see into the pipe. It was still half-full with water. But he said he was encouraged that the problem might be solved relatively soon because the sleeves could be fixed more easily than the steel pipe.

“In a sense, it’s the most vulnerable point in the system,’’ said Tom Baron, an independent water systems engineer who formerly worked for the water authority.

Officials said that if they cannot repair the pipe with a temporary patch, a new custom-made pipe might have to be built. Officials had been building a backup system for Greater Boston’s drinking water system, but it is three or four years from completion. Before water was shut off to the ruptured pipe yesterday, brown water had been roaring from a massive crater in the ground, sending more than 8 million gallons an hour rushing down a hill into the nearby Charles River.

The state Department of Conservation and Recreation said the millions of gallons pouring into the river would not be a problem, even as the river’s elevation rose by 8 inches in some locations and its flow nearly doubled in a matter of hours.

“Our dams can handle this,’’ said Wendy Fox, a spokeswoman for the agency.

But Nigel Pickering, senior engineer and watershed modeler for the Charles River Watershed Association, warned that the water entering the river from the break could stir up sediment, harming fish. Laskey said it did not appear that sediments had been kicked up as of last night.

Authorities said that contractors and engineers would know more about what went wrong today, when they have a chance to better inspect the pipe.

“I really don’t want to speculate,’’ Laskey said at the scene of the break. “We’ve got to get there to know.’’

Not every community was affected. Cambridge, for example, has its own water supply.

And water authority spokeswoman Ria Convery said that the authority, working with the Boston Water and Sewer Commission, was able to reconfigure the water pipe lines in the Longwood Medical Area. That temporary fix allowed four major hospitals in the area — Children’s Hospital, the Dana-Farber Cancer Institute, Beth Israel Deaconess Medical Center, and Brigham and Women’s Hospital — to go about business as usual, without having to resort to bottled water, she said.

Eric Moskowitz and John M. Guilfoil of the Globe staff contributed to this report.